On 1st January 1873, Dowlais became the western terminus of the London & North Western Railway's branch from Abergavenny, the route having being extended piecemeal since the first section opened to Brynmawr in 1862. The company's next objective was to secure a route into nearby Merthyr Tydfil. Faced with the threat of unwelcome competition from a railway giant, the Brecon & Merthyr Railway accepted the L&NWR's offer of half the original construction costs for a joint stake in its line running around the west side of the town to enter from the south. To make sense of this arrangement, the L&NWR sought Parliamentary approval for a link between its own line at Penywern Junction and the B&M near Vaynor, authority for which was granted in July 1874.

From its northern end, the tunnel curves to the south for 170 yards on a radius of about 14 chains, giving way to a long, straight section. The lining comprises masonry sidewalls - with refuges - and a brick arch. Rows of cable hangers are in situ on the east side, just above springing level.
Soon encountered is a series of five braced iron ribs, fashioned from bullhead rail and standing on stout timber baulks. These were installed to provide support to the lining. Close by is a section of arch where the west-side haunch has been significantly flattened due to ground pressure. A considerable amount of water penetrates the brickwork near this point.

The three shafts remain open, the middle one having lost its original pepperpot which now languishes in the tunnel. Above ground, the protection walls have distinctive battered sides with raised square labels under the top band of brickwork. Above this is a sandstone coping carrying the domed iron grilles. The centre and south-east shafts are Grade II listed.
The stone-built north portal - bedecked in moss and saplings - looks out towards Morlais Tunnel Junction. A long retaining wall supports the west side of the approach cutting; the other has just a short triangular wing wall. The south portal is more striking but has suffered due to the effects of time and vandalism. Its headwall is now consumed by ivy but once featured a stepped parapet with dressed copings; the commemorative datestone is/was centred above the crown of the arch. If it's still there, foliage now obscures it. The face of five brick rings are apparent, with three of these being curved to add attractive architectural detailing. Closed 1958